Engines may be operated in a deceleration fuel shut-off (DFSO) condition to save fuel. Therein, fuel injectors are turned off while air continues to flow through the cylinders, and the engine is spun down with fuel disabled. Once the engine speed has sufficiently dropped, or in response to an increase in torque demand, the DFSO conditions may be exited wherein fuel delivery is resumed. During the DFSO exit, a torque bump may occur when the engine torque goes from negative (fuel shut off) to positive (fuel on). Further, when exiting from the DFSO condition, the engine may be operated with a rich air fuel ratio (AFR) to increase efficiency of exhaust catalysts that may have been saturated with oxygen when fuel was disabled. Due to the rich AFR, the engine torque output may increase, further exacerbating the torque bump. This can cause an undesirable and noticeable torque bump that passes through the drivetrain and can be perceived by the driver.
Example approaches to reduce torque bumps include changing a fuel injection mode. For example, in engines configured with direct fuel injection, fuel may be delivered via an intake stroke direct injection mode (also referred to as homogeneous mode) and/or a compression stroke direct injection mode (also known as stratified mode). In the intake stroke direct injection (DI) mode, the combustion chambers contain a substantially homogeneous mixture of air and fuel. In the compression stroke DI mode, the combustion chambers contain stratified layers of different air/fuel mixtures including a stoichiometric air/fuel mixture nearer the spark plug and lower strata containing progressively leaner air/fuel mixtures. Engine operation may be controlled when switching between the stratified and the homogeneous mode to deliver the demanded torque without adversely affecting driveability.
One example approach is shown by Yamada et al. in U.S. Pat. No. 6,240,354. Therein, to increase homogeneous charge and torque output, fuel is injected twice: once during the intake stroke and again during the compression stroke to reduce torque fluctuations.
However, the inventors herein have recognized potential issues with such an approach. As one example, using two injections, one during the intake stroke and the other during the compression stroke, results in a combustible mixture layer adjacent to a spark plug, while the rest of the combustion chamber contains a lean mixture. This generates a weak stratified charge combustion and may not be able to provide a large enough initial torque during DFSO exit conditions. As a result, the engine may stall during the DFSO exit. In addition, using two injections during a DFSO exit may require additional control and complexity to ensure accurate control of timing between the injections.
In one example, the issues described above may be addressed by a method for controlling engine torque, the method comprising: during an exit from a deceleration fuel shut-off (DFSO) condition, fueling an engine via a compression stroke direct injection (DI) at a first separation from a spark event until an engine torque reaches a first threshold, then increasing a separation between the compression stroke DI and the spark event until the engine torque reaches a second, higher threshold, and thereafter transitioning to engine fueling via an intake stroke DI. Herein, the first threshold may be a peak engine output torque that is determined prior to the DFSO exit, and may be sufficient to give the initial increase in torque that is needed when the engine exits from a DFSO condition. In this way, engine stalls may be avoided.
As one example, during selected engine operating conditions (e.g., light engine load conditions), an engine may be fueled using compression stroke direct injection to provide a stratified charge distribution inside a cylinder. When fueling using the compression stroke direct injection, a controller may learn a separation between a timing of the compression stroke direct injection and a spark event that generates a peak engine output torque (for the given conditions), the peak engine output torque then saved in the controller's memory as a first torque threshold. During a subsequent exit from a DFSO condition, the controller may fuel the engine using compression stroke direct injection while applying the learned separation between compression stroke direct injection timing and spark timing. Once the engine reaches the peak engine output torque, the separation between the compression stroke direct injection timing and the spark timing may be increased until a second torque threshold, higher than the first torque threshold, is reached. Thereafter the engine may be transitioned to being fueled via intake stroke direct injection.
In this way, an engine may be able to produce a previously learned peak engine output torque during an exit from DFSO conditions with reduced likelihood of stalls. The technical effect of increasing the separation between the timing of the compression stroke direct injection and the spark timing after the peak engine output torque is reached is that the resulting drop in engine torque may be used to offset the increase in engine torque that occurs as a result of operating the engine with a rich air fuel ratio (AFR) during a DFSO exit. Consequently, instead of encountering a noticeable torque bump, a gradual increase in torque is provided through the driveline which may not be objectionable to the driver. By transitioning the engine from being fueled via compression stroke direct injection to intake stoke direct injection after the engine torque has exceeded a threshold, the engine may be operated with a more homogeneous air/fuel mixture which is maintained at or near stoichiometry, thus enabling cleaner combustion and producing lower emissions. In this way, the engine may be transitioned out of DFSO with a smoother torque profile, thereby enhancing drivability.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.